This attack alwyas does a damage of 1, i casted it 8 times over the sewer rats, out of 5 times it hit, it always did 1 damage.
Acording to the description, this attack does 1d4 damage per class level, since this is a shifter ability, and the imp has no shifter levels, it is treated as no damage, i imagine, and therefore deals 1 always.
This could be fixed by making the attack use character levels, rather than class levels (i think), additionally, a quick fix might be incresing the imp's wisdom, since the attack adds +1d4 fire damage per wis modifier. A wisdom of 16 would have the imp deal 3d4 damage.